There is now a substantial body of evidence indicating that reactive oxygen metabolites (=ROM) and PMNs mediate ischemia/reperfusion (I/R)-induced tissue damage (Korthuis and Granger, 1986; Hearse et al., 1986).
Several studies have indicated that certain enzymes may be involved in the production in vivo of oxygen radicals like O.sub.2.sup.- and HO' and thereby being import mediators of I/R-induced tissue damage. Most of the interests has been focused on xanthine oxidase, an enzyme existing in intestinal epithelia and vascular endothelium. Due to the central role that ROM has in I/R-induced injury, it has been proposed to administer substances acting as oxygen radical scavangers and agents inhibiting radical producing enzymes in order to prevent this type of injury. Positive effects have been achieved but they have not been satisfactory.
The PMNs have been suggested to cause injury following ischemia and reperfusion in the myocardium (Romson et al., 1983; Engler et al., 1983; Schmid-Schoenbein & Engler, 1987) hemorrhagic shock induced gastric mucosal injury (Smith et al., 1987), skeletal muscle (Bagge et al., 1980), and the brain (Grogaard et al., 1987). The mechanism by which the PMNs cause I/R injury has been suggested to be mechanical plugging of capillaries giving as a consequence a reduced blood flow (Schmid-Schoenbein & Engler, 1987; Smith et al., 1987) and/or release of tissue damaging substances like reactive oxygen metabolites (Korthuis & Granger, 1986), cationic proteins (Henson & Johnston, 1987), and proteases (Harlan, 1985) respectively.
The cause of PMN infiltration to the ischemic area is more open to debate. Dead or damaged tissue is known to have chemotactic activity itself (O'Flaherty & Ward, 1979) or indirectly by activating the complement system. Further, during hypoxia xanthine dehydrogenase is converted to xanthine oxidase, which further reacts with hypoxanthine to form xanthine and urea, thereby producing superoxide (Granger et al., 1981). Superoxide can bring about oxidative changes in arachidonic acid resulting in the appearance of a chemotactic lipid (Perez et al., 1980; Petrone et al., 1980). The involvement of xanthine oxidase in I/R injury is supported by the beneficial effect xanthine oxidase inhibitors (i.e. allopurinol) have on I/R tissue damage (Grisham et al., 1986), thus supporting an important role for a superoxide induced chemotactic lipid.
After the finding that adherence of PMNs is critical for the development of I/R-induced injury much interest has been focused on leukocyte-endothelial vascular interactions. This phenomenon is very complex and has been reviewed by several authors (e.g. Harlan, 1985).
The membrane leukocyte adhesion complex (LAC) is critical for the in vitro (Harlan et al., 1985) and in vivo (Arfors et al., 1987) adherence of stimulated PMNs to endothelial cells. Patients deficient in LAC, or parts thereof, have recurrent infections without pus formation, and their PMNs do not adhere to endothelium in vitro.
LACs consists of three subunits: LFA-1 being expressed on lymphocytes and monocytes, Mac-1 on granulocytes and monocytes, and p150.95 on macrophages and monocytes. Each subunit consists of one common beta-chain (CD18) and an alpha-chain that is unique for each of the three subunits (CD11a, CD11b, and CD11c, respectively). The LAC complex has been extensivel studied (Sanches-Madrid et al., 1983) and designated LFA-1/Mac-1/p150.95 (Bernstein and Self, 1985). Several monoclonal antibodies (MoAb) have been raised against its different epitopes. Some of them such as MoAb 60.3, IB4, CL54 etc have been shown to block leukocyte adherence to endothelial cells both in vivo and in vitro. In particular MoAbs that inhibit leukocyte adherence bind to the beta-chain, but the alpha-chain may also be involved in the adherence by providing part of the binding epitope on LAC. The epitope responsible for adherence may be exposed on the leukocytes due to a conformation change in the beta-chain and possible also in the alpha-chain. The most extensively studied MoAb having an inhibitory effect on leukocyte adherence to endothelial cells is MoAb 60.3, and addition of MoAb 60.3 to normal PMNs induces in vitro defects in PMN spreading, adherence, and chemotaxis similar to those observed in LAC-deficient patients. Furthermore, MoAb 60.3 has been shown to inhibit PMN accumulation and plasma leakage in rabbit skin inflammatory lesions. The above results with MoAb 60.3 have been published (Arfors et al., 1987; Beatty et al., 1984; Harlan et al., 1985; Diener et al., 1985; Pohlman et al., 1986; and Wallis et al., 1986). IB4 has been described (Wright et al., 1983; van Voorhis et al., 1983) as well as CL54.